1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly, to a CMOS image sensor and a method for fabricating the same. The CMOS image sensor improves photosensitivity by providing a color filter layer having a microlens function and by eliminating the need for a microlens photoresist layer.
2. Discussion of the Related Art
An image sensor is a semiconductor device for converting an optical image into an electrical signal and may be broadly categorized as a charge-coupled device (CCD) or a complementary metal-oxide-semiconductor (CMOS). A CMOS image sensor comprises a photosensor for receiving light and a logic circuit for converting the received light into an electrical signal. To improve photosensitivity, the fill factor of the CMOS image sensor should be improved. That is, the area occupied by the photosensor should be increased with respect to the overall area of the entire CMOS image sensor. There is a limit to increasing the photosensor area since the CMOS image sensor itself has a finite size.
Alternatively, photosensitivity can be effectually improved by concentrating or focusing incident light onto the photosensor. Incident light on the CMOS image sensor outside the area of a photosensor is redirected to land on the photosensor. To achieve this in the related art, a microlens that corresponds to the photosensor is provided. The microlens is typically on a color filter array.
As shown in FIG. 1, a CMOS image sensor according to the related art includes a semiconductor substrate 10, an insulating interlayer 11, a color filter layer 12 formed on the insulating interlayer 11, a planarization layer 13 formed on the color filter layer 12, and a plurality of microlenses 14 formed on the planarization layer 13. The semiconductor substrate 10 includes a lower layer (not shown) of metal lines interconnected with a plurality of photodiodes generate electrical charges according to an amount of incident light. Furthermore, and the entire surface of the lower layer is covered with the insulating interlayer 11.
The color filter layer 12 is comprised of an array of red, green, and blue patterns for filtering light according to wavelength or color. Each microlens 14 is formed in a convex shape having a predetermined curvature and height for focusing light onto a corresponding photodiode through one of the patterns of the color filter layer 12. Generally, the microlenses 14 are formed of a polymer-based resin, typically, a photoresist, and can be completely formed by deposition, patterning and reflowing processes. The processes of microlens formation and color filter array formation are directly related to the color characteristics of the CMOS image sensor.
In a method for fabricating the CMOS image sensor described above, after a basic lens array pattern including a plurality of lens formation blocks is formed, each block of the lens pattern is imparted with its predetermined curvature by reflowing. Each lens formation block has a rectangular section and is separated by a fixed interval. In the reflowing process, however, it is difficult to maintain the proper interval between lenses. Moreover, due to characteristics of the material used for forming the microlens, e.g., photoresist, some of the energy of the incident light is lost or attenuated during transmission to the lower layers. The amount of light loss varies depending on wavelength. That is, about 2˜5% of the light is not transmitted by the microlens material. Thus, the photosensitivity of the image sensor is reduced accordingly.